1,4,7,10-Tetraazacyclododecane-1,4,7-triacetic acid,10-[[2,3-bis(ethoxycarbonyl)pyrazino[2,3-f][1,10]phenanthrolin-7-yl]methyl]-, 1,4,7-tris(1,1-dimethylethyl) ester [ 916671-23-3 ]